Optical device and a making method thereof

ABSTRACT

After a wide-band DCF is wound around a bobbin to form an optical fiber coil  32,  the latter is removed from the bobbin and placed into a bundle state (the state where the increase in transmission loss in the wavelength band of 1.55 μm caused by distortions in winding is reduced by 0.1 dB/km or more) released from distortions in winding. A resin  42  is used as a coil-tidying member so as to secure the optical fiber coil  32  to a storage case  40  at four positions. Both ends of the optical fiber coil  32  are connected to pigtail fibers at fusion-splicing parts  44,  respectively. Even when the storage case  40  is closed with a lid after the optical fiber coil  32  is secured to the storage case  40  with the resin  42,  there remain interstices within the bundle of the optical fiber coil  32  and a space between the optical fiber coil  32  and the storage case  40.  As a result, even when the optical fiber coil  32  in a bundle state is accommodated in the storage case  40,  transmission loss and the like would not increase.

RELATED APPLICATION

[0001] The present application is a continuation-in-part application of U.S. patent application Ser. No. 09/240,690 filed on Feb. 2, 1998 now pending.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a coiled optical assembly made of a long optical fiber and a fabricating method for the same; and, in particular, to a dispersion compensator for reducing the wavelength dispersion of an optical fiber transmission line in the wavelength band of 1.55 μm, and a fabricating method for the same.

[0004] 2. Related Background Art

[0005] Long-distance, large-capacity transmission is possible in the wavelength band of 1.55 μm by means of an optical amplifier operable in the wavelength band of 1.55 μm utilizing an optical fiber doped with erbium (Er) which is a rare-earth element. However, when transmission is carried out in the wavelength band of 1.55 μm by using a single-mode optical fiber whose zero-dispersion wavelength is in the 1.3-μm band (1.3 SMF), which is employed in conventional optical transmission lines, then a large wavelength dispersion occurs and distorts optical signals, thereby deteriorating the signal quality. As a consequence, when carrying out transmission in the 1.55-μm band with the use of 1.3 SMF, its wavelength dispersion is required to be kept low. Known as one of techniques therefor is a method using a dispersion-compensating optical fiber (DCF) having a large wavelength dispersion with a polarity opposite to that of the dispersion of 1.3 SMF, so as to cancel the wavelength dispersion in the 1.55-μm band.

[0006] In a technique employed for carrying out such dispersion compensation with DCF in a conventional long-distance transmission line, a dispersion compensator having a compact size in which a long DCF is wound around a bobbin is installed at every repeater station.

SUMMARY OF THE INVENTION

[0007] Nevertheless, since a wide-band DCF has a large bending loss in general, it may yield a large transmission loss in the 1.55-μm band, i.e., transmission wavelength band, when formed into a coil having a small diameter. This bending loss can be reduced when the coil has a larger diameter so that its number of turns is reduced. Increasing the coil diameter, however, is unfavorable in that the dispersion compensator accordingly becomes larger.

[0008] Also, the dispersion compensator is often used together with an optical amplifier using an erbium-doped optical fiber. In this case, the temperature of the dispersion compensator increases due to the heat from a pumping laser within the optical amplifier, so that the bobbin may thermally expand. As a result, distortions may occur in the wound wide-band DCF, thereby increasing the transmission loss. Using a material with less thermal expansion in the bobbin can reduce the transmission loss in such a high-temperature environment. However, materials having low coefficients of thermal expansion, such as silica glass, ceramics, special alloys, and the like, are hard to process or expensive.

[0009] Therefore, it is an object of the present invention to provide an optical assembly accommodating therein a long optical fiber in a compact fashion, which yields less increase in transmission loss upon such bending or heat; and a method of making the same.

[0010] In order to achieve the above-mentioned object, the inventors carried out various studies using the wide-band DCF, results of which will be discussed here.

[0011]FIG. 1 is a sectional view of a DCF constituting an optical fiber coil studied. As shown in FIG. 1, the DCF employed has an optical fiber 11 made of glass and two coating layers 13, 15, each made of a resin, formed around the optical fiber 11. FIG. 2 shows the refractive index profile of this DCF. The glass portion 11 is a double-cladding type DCF whose core portion has a diameter a of 2.65 μm, depressed cladding portion has a diameter b of 7.58 μm, and outside diameter c is 100 μm. The primary coating layer 13 has a thickness d of 20 μm. The secondary coating layer 15 has a thickness e of 20 μm. The outside diameter f of the fiber is 180 μm. The relative refractive index differences Δ+, Δ− of the core portion and depressed cladding portion with respect to the refractive index of the outer cladding portion were set to 2.1% and −0.4%, respectively. At 20° C., the Young's modulus of the primary coating layer 13 was 0.06 kgf/mm², and that of the secondary coating layer 15 was 65 kgf/mm². The wavelength dispersion and wavelength dispersion slope of this DCF were −100 ps/nm/km and −0.29 ps/nm²/km at the wavelength of 1.55 μm, respectively, whereas its transmission loss was 0.40 dB/km.

[0012]FIG. 3 is a perspective view of a take-up bobbin 2 used for producing the optical fiber coil. Around the bobbin 2 made of aluminum having a body portion 20 with a diameter g of 100 mm, flanges 21 with a diameter of 200 mm, and a winding width k of 18 mm, the above-mentioned DCF having a fiber length of 10 km was wound at a winding pitch of 0.4 mm with a take-up tension of 40 gf, so as to produce the optical fiber coil.

[0013] The respective transmission characteristics of thus obtained optical fiber coil (type 1) wound around the bobbin, the optical fiber coil (type 2) loosened into a bundle form after being removed from the bobbin, and the optical fiber coil (type 3) obtained after that of type 1 had been subjected to a predetermined heat treatment were measured and compared with each other.

[0014]FIG. 4 shows the heat cycle of the heat treatment applied to the optical fiber coil of type 3. In this heat treatment, the optical fiber coil of type 1 was left for 1 hour at a temperature of 20° C., subsequently the temperature was raised at a rate of 1° C./minute until it reached 80° C., at which the optical fiber coil was left for 1 hour, and then the temperature was lowered at a rate of 1° C./minute until it reached −40° C., at which the optical fiber coil was left for 1 hour. After this cycle was repeated once again, the optical fiber coil was finally maintained at 20° C. and left for 2 hours.

[0015]FIG. 5 is a graph comparing, at each wavelength, the transmission loss values of the optical fiber coils of types 1 and 2 with those of the optical fiber before being wound up. In the optical fiber coil of type 1, a large transmission loss (1.7 dB at 1.55 μm) occurred in the 1.55-μm band, i.e., transmission wavelength band, and the transmission loss became greater as the wavelength was longer. It is due to the microbend loss occurring when the optical fiber is bent with a small curvature. By contrast, this microbend loss substantially disappeared from the optical fiber coil of type 2. From these facts, the inventors have found that the transmission loss generated upon the winding of a coil is mainly caused by distortions in winding due to a multiplex winding, e.g., lateral pressures applied to each fiber from its adjacent fibers, which cause the optical fiber to bend, thereby generating a microbend loss upon coil winding. Hence, the inventors have concluded that an optical fiber coil having a low transmission loss can be produced if these lateral pressures are eliminated.

[0016] When the optical fiber coil of type 2 was heated to 70° C. and then its transmission loss was measured at a wavelength of 1.55 μm, the measured value was greater than that at 20° C. by 0.06 dB. This minute amount of change in transmission loss value is on a par with the value of 1.3 SMF reported in a literature (Tanaka et al., “TEMPERATURE DEPENDENCE OF INTRINSIC TRANSMISSION LOSS FOR HIGH SILICA FIBER,” European Conference on Optical Communication, pp. 193-196, 1987). Therefore, this transmission loss is considered to be the temperature-dependent loss inherent in the optical fiber, which is a value irrelevant to the lateral pressures.

[0017]FIG. 6 is a graph comparing, at each wavelength, the transmission loss values of the optical fiber coils of types 1 and 3 with those of the optical fiber before being wound up. In the optical fiber coil of type 3, the amount of change in loss was improved over that of the optical fiber coil of type 1, whereby its amount of change in transmission loss at a wavelength of 1.55 μm was 0.25 dB. Though the amount of change in transmission loss of this optical fiber coil at a wavelength of 1.55 μm further increased by 0.24 dB when heated to 70° C., it was still lower than that of the optical fiber coil of type 1 at 20° C., i.e., 1.7 dB.

[0018] Thus, the inventors have found that subjecting a coil to a heat treatment can reduce its lateral pressures and also lower the temperature dependence of its amount of change in transmission loss.

[0019] Therefore, the present invention is configured as follows.

[0020] The coiled optical assembly made of a long optical fiber in accordance with the present invention comprises an optical fiber coil in which the long optical fiber is formed into a coil and adjusted into a state where the amount of increase in transmission loss in a predetermined wavelength band upon coiling is reduced by 0.1 dB/km or more, and a storage case accommodating the optical fiber coil.

[0021] Since the optical fiber coil is accommodated in the storage case in a state where the amount of increase in transmission loss in a predetermined wavelength band upon coiling is reduced by 0.1 dB/km or more so as to substantially release distortions in winding, the lateral pressure applied by each layer of the optical fiber to another layer of the optical fiber due to multiplex winding of the long optical fiber is alleviated. Also, when wound around a bobbin, the optical fiber is less likely to be influenced by the thermal expansion of the bobbin even in a high-temperature environment.

[0022] When the long optical fiber has a wavelength dispersion and a wavelength dispersion slope which have polarities opposite to those of the wavelength dispersion and wavelength dispersion slope of the optical fiber constituting a transmission line, respectively, then the coiled optical assembly functions as a dispersion compensator for reducing the wavelength dispersion in a predetermined wavelength band. As a consequence, a dispersion compensator having a favorable characteristic can be obtained.

[0023] The storage case may accommodate the long optical fiber in a bundle form. When the coil-shaped optical fiber is loosened into a bundle form and accommodated in the storage case, then its distortions in winding can substantially be released in a simple manner.

[0024] Alternatively, the optical fiber coil may be wound around a bobbin. In this case, the lateral pressures applied to the optical fiber from its adjacent layers of optical fiber or the bobbin are alleviated, and the optical fiber coil is accommodated so as to be free from the influence of the thermal expansion of the bobbin even when placed in a high-temperature environment.

[0025] The bobbin may be made of a metal. In this case, by forming the optical fiber coil in a state where the bobbin is heated and expanded, and then cooling and shrinking the bobbin, the optical fiber coil can be adjusted into a state where distortions in winding are substantially released.

[0026] When the winding pitch of the optical fiber coil is set to a value at least twice as large as the diameter of the long optical fiber, then mode coupling occurs between orthogonal polarization modes, whereby polarization mode dispersion (PMD) can be reduced.

[0027] The diameter of the optical fiber coil may be 100 mm or less in its smallest portion. In such a state which is substantially free of distortions in winding, it is possible to produce an optical assembly using a small-sized optical fiber coil having a diameter of 100 mm or less, which has conventionally been difficult.

[0028] Preferably, the long optical fiber has a coating layer on the outer peripheral surface thereof. Preferably, this coating layer is the one in which a primary coating layer made of a coating material having a Young's modulus of at least 0.03 kgf/mm² but not greater than 0.15 kgf/mm² and a secondary coating layer made of a coating material having a Young's modulus of at least 50 kgf/mm² but not greater than 100 kgf/mm² are laminated, or a coating layer made of a coating material having a Young's modulus of at 1 kgf/mm² but not greater than 120 kgf/mm². Preferably, the coating layer has a thickness of at least 20 μm but not greater than 70 μm.

[0029] When such a coating layer is provided, then it becomes easier to coil the long optical fiber in a state substantially released from distortions in winding.

[0030] The optical assembly of the present invention may further comprise a coil-tidying member which secures the optical fiber coil to the storage case or bobbin, thereby preventing the optical fiber coil from becoming disordered in winding.

[0031] When the coil-tidying member secures the optical fiber coil to the storage case or bobbin, then, even when the optical fiber coil is accommodated in the state substantially released from distortions in winding, it is possible to prevent the optical assembly from vibrating upon transportation or the like, the optical fiber from breaking upon impacts, the wound optical fiber coil from becoming disordered, and the transmission loss from thereby increasing.

[0032] The coil-tidying member may be formed by a resin which secures the optical fiber coil to the storage case or bobbin at a plurality of positions. Alternatively, it may be a cushion material securing the optical fiber coil to the storage case. The coil-tidying member may further comprise a filler for securing and holding the optical fiber coil. As the filler, a thermosetting or UV-curable silicon gel having a Young's modulus of 0.05 kg/mm² or less upon curing or a jelly-like admixture having a high viscosity is preferable.

[0033] When the optical fiber coil is thus secured to the storage case or bobbin by means of the coil-tidying member, it is possible to easily prevent the optical assembly from vibrating upon transportation or the like, the optical fiber from breaking upon impacts, the wound optical fiber coil from becoming disordered, and the transmission loss from thereby increasing.

[0034] Also, a cushioning filler may fill gaps between turns of the optical fiber constituting the optical fiber coil. As this filler, a thermosetting or UV-curable silicon gel having a Young's modulus of 0.05 kg/mm² or less upon curing or a jelly-like admixture having a high viscosity is preferable.

[0035] When the cushioning filler fills gaps between turns of the optical fiber, then the lateral pressures generated by the turns of the optical fiber acting on each other are alleviated, whereby distortions in bending are suppressed. Further, this filler can easily prevent the optical assembly from vibrating upon transportation or the like, the optical fiber from breaking upon impacts, the wound optical fiber coil from becoming disordered, and the transmission loss from thereby increasing.

[0036] On the other hand, the fabricating method for a coiled optical assembly made of a long optical fiber in accordance with the present invention comprises a coil making step of winding the long optical fiber around a bobbin so as to make an optical fiber coil; and an adjustment step of adjusting the optical fiber into a state where the amount of increase in transmission loss in a predetermined wavelength band upon this coil making step is reduced by 0.1 dB/km or more.

[0037] Thus, as the optical fiber is placed into a state where the amount of increase in transmission loss in a predetermined wavelength band upon this coil making step is reduced by 0.1 dB/km or more in the adjustment step so as to substantially release distortions in winding, the coiled optical assembly in accordance with the present invention can be produced.

[0038] For example, this adjustment step can employ any of: (1) a step of holding, after the coil making step, the optical fiber coil at at least one of temperatures which are lower and higher than the temperature in the coil making step, respectively; (2) a step of vibrating the bobbin wound with the optical fiber coil after the coil making step; and (3) a step of removing the optical fiber coil from the bobbin and loosening thus removed optical fiber coil into a bundle form.

[0039] In the step (1), which is derived from the above-mentioned finding, the coil is subjected to a heat treatment, so as to reduce lateral pressures, whereby the coil is adjusted into the state where distortions in winding are substantially released. In the step (2), the bobbin is vibrated so as to homogenize the winding state of the coil, thereby making the lateral pressures uniform and adjusting the optical fiber coil into the state substantially released from distortions in winding. In the step (3), the coil is loosened into a bundle form, so as to be adjusted in the state substantially released from distortions in winding.

[0040] The method may further comprise a filling step of accommodating the optical fiber coil in a storage case and filling the storage case with a cushioning filler. When the storage case is thus filled with the cushioning filler, the optical fiber coil can be secured and held without excess lateral pressures being applied thereto.

[0041] The method may further comprise a lubricant coating step of coating the bobbin with a lubricant prior to at least one of the coil making step and adjustment step. This is effective in that the friction between the bobbin and the optical fiber coil can be reduced, whereby it is possible to reduce lateral pressures and enhance the elimination of distortions in winding upon the adjustment step.

[0042] In another aspect, the fabricating method for a coiled optical assembly in accordance with the present invention comprises a coil making step of forming an optical fiber coil by winding a long optical fiber around a bobbin in a state where the amount of increase in transmission loss in a predetermined wavelength band upon coiling is reduced by 0.1 dB/km or more. Preferably, this coil making step comprises at least one of (1) a step of winding the long optical fiber around the bobbin in a state where at least one of the long optical fiber and the bobbin is maintained at a predetermined temperature not lower than 60° C.; and (2) a step of winding the long optical fiber around the bobbin at a take-up tension of 50 gf or less.

[0043] The step (1) can alleviate lateral pressures which may occur after the bobbin or optical fiber is cooled. The step (2) can form a coil in a state substantially released from distortions in winding.

[0044] In still another aspect, the fabricating method for a coiled optical assembly comprises a coil making step of winding a long optical fiber around a bobbin so as to make an optical fiber coil; and a body diameter changing step of substantially reducing, after the coil making step, the diameter of the bobbin from the diameter thereof in the coil making step, so that the optical fiber is adjusted into a state where the amount of increase in transmission loss in a predetermined wavelength band upon winding is reduced by 0.1 dB/km or more.

[0045] When the body diameter is thus substantially reduced in the body diameter changing step, so as to form an optical fiber coil in a bundle form in which a wide-band DCF is loosened, then the lateral pressures generated due to a multilayer of winding are alleviated, whereby the optical fiber coil that is not influenced by thermal expansion of the bobbin even when placed in a high-temperature environment can reliably be made.

[0046] Further, the method of making an optical assembly in accordance with the present invention may comprise a filler coating step of coating an outer periphery of a long optical fiber with a cushioning filler, and a coil making step of winding the long optical fiber, whose outer periphery is coated with the filler, around a bobbin, so as to make an optical fiber coil; or a coil making step of making an optical fiber coil while coating, simultaneously with winding the long optical fiber around a bobbin, the surface of the bobbin for winding the long optical fiber and the outer periphery of the wound long optical fiber with a cushioning filler.

[0047] In this manner, since the cushioning filler is disposed between turns of the optical fiber and between the optical fiber and the bobbin, the lateral pressures applied to the optical fiber are alleviated, whereby the optical fiber coil that is not influenced by thermal expansion of the bobbin even when placed in a high-temperature environment can securely be made.

[0048] The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.

[0049] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050]FIG. 1 is a sectional view of a DCF constituting an optical fiber coil used for a study prior to the present invention, whereas

[0051]FIG. 2 is a view showing its refractive index profile;

[0052]FIG. 3 is a perspective view of a take-up bobbin used for making the optical fiber coil;

[0053]FIG. 4 is a chart showing the heat cycle of a heat treatment applied to an optical fiber coil produced in a comparative experiment;

[0054]FIGS. 5 and 6 are graphs comparing amounts of change in transmission loss of optical fiber coils produced in comparative experiments;

[0055]FIG. 7 is a schematic view showing a dispersion compensator in accordance with a first embodiment of the present invention, whereas

[0056]FIG. 8 is a schematic view showing a moment at which an optical fiber coil is accommodated in a storage case in this dispersion compensator;

[0057]FIG. 9 is a side view showing a dispersion compensator in accordance with a second embodiment of the present invention, whereas

[0058]FIG. 10 is a vertical sectional view thereof;

[0059]FIG. 11A is an exploded view of a bobbin in the second embodiment,

[0060]FIG. 11B is a side view of a wedge member thereof,

[0061]FIG. 11C is a sectional view of the bobbin taken along a plane including the center axis of the bobbin,

[0062]FIG. 12A is a top plan view of a support member of the bobbin,

[0063]FIG. 12B is a transverse sectional view of the support member, and

[0064]FIG. 12C is an enlarged view of a groove portion of the support member;

[0065]FIGS. 13A to 13D are views for explaining a mechanism for reducing the body diameter of the bobbin after a DCF is wound therearound;

[0066]FIG. 14 is an exploded perspective view of a dispersion compensator in accordance with a third embodiment of the present invention, whereas

[0067]FIGS. 15 and 16 are perspective views of cushion materials thereof;

[0068]FIG. 17 is a perspective view showing a dispersion compensator in accordance with a fourth embodiment of the present invention, whereas

[0069]FIG. 18 is a perspective view of a modified example thereof;

[0070]FIG. 19 is a sectional view showing a dispersion compensator in accordance with a fifth embodiment of the present invention, whereas

[0071]FIG. 20 is a sectional view of a modified example thereof;

[0072]FIG. 21 is an enlarged sectional view showing a dispersion compensator in accordance with a sixth embodiment of the present invention, whereas

[0073]FIGS. 22 and 23 are views for explaining methods of making the same;

[0074]FIG. 24 is a perspective view showing a dispersion compensator in accordance with a seventh embodiment of the present invention;

[0075] FIGS. 25 to 27 are charts each showing the tension dependence of the transmission loss value of an optical fiber coil;

[0076]FIG. 28 is a chart showing the winding pitch dependence of PMD in an optical fiber coil;

[0077]FIG. 29 is a chart showing how temperature changes in a heat cycle applied to a wound coil, whereas

[0078]FIG. 30 is a chart showing how the transmission loss value changes over time when the coil is subjected to this heat cycle;

[0079]FIG. 31 is a chart showing how temperature changes in another heat cycle applied to the wound coil;

[0080]FIG. 32 is a view showing a refractive index profile of another DCF;

[0081]FIG. 33 is a schematic view showing how a DCF is wound around a coil while being heated; and

[0082]FIGS. 34 and 35 are configurational views of optical transmission systems each using a dispersion compensator in accordance with the present invention, whereas

[0083]FIGS. 36 and 37 are configurational views of optical amplifiers each having a dispersion compensator;

[0084]FIG. 38 is a plan view to show the eighth embodiment of the dispersion compensator according to the present invention in the state in which the container lid is taken off.

[0085]FIG. 39 and FIG. 40 are diagrams each of which shows an example in which the optical fiber coil is placed around the center cylinder.

[0086]FIG. 41 is a diagram to show an aligned state of the optical fiber in the optical fiber coil formed by winding the optical fiber around the center cylinder.

[0087]FIG. 42 is a diagram to show an example in which a plurality of optical fiber coils are arranged concentrically.

[0088]FIG. 43 is a diagram to show a layout example in which a plurality of optical fiber coils are stacked along the center axis of the coil shape.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0089] In the following, preferred embodiments of the present invention will be explained in detail with reference to the accompanying drawings. To facilitate the comprehension of the explanation, the same reference numerals denote the same parts, where possible, throughout the drawings, and a repeated explanation will be omitted. Though the following explanations mainly relate to embodiments of dispersion compensators in which optical fiber coils are made of a wide-band DCF (hereinafter simply referred to as DCF), the present invention is also applicable to optical assemblies which form optical fiber coils by using other long optical fibers.

[0090] First Embodiment

[0091] To begin with, a dispersion compensator in accordance with a first embodiment will be explained. FIG. 7, which illustrates the dispersion compensator in accordance with the first embodiment, is a sectional view showing an optical fiber coil 32 accommodated in a storage case 40.

[0092] As shown in FIG. 7, the optical fiber coil 32 bundled in a state substantially released from distortions in winding is accommodated in the storage case 40 and secured thereto at four positions with the aid of resin pieces 42. Both ends of the optical fiber coil 32 are connected to pigtail fibers 45 at fusion-splicing parts 44, respectively.

[0093] Here, the state substantially released from distortions in winding refers to a state where the increase in transmission loss in the wavelength band of 1.55 μm caused by the distortions in winding is suppressed to 0.1 dB/km or less. It is due to the fact that the increase in transmission loss of the optical fiber coil 32 in a loosened state removed from the bobbin is substantially eliminated as shown in FIG. 4 and, as the distortions in winding are eliminated, their resulting transmission loss also disappears.

[0094] In order to restrain the transmission loss value of the optical fiber coil from increasing and suppress its temperature dependence, the resin preferably has a low coefficient of thermal expansion and a low coefficient of contraction after its coating, while exhibiting a sufficient adhesive property. An example of such a resin is KE45T, which is a silicone resin manufactured by Shin-Etsu Chemical Co., Ltd. Preferably, at least four securing parts are formed by the resin at substantially identical intervals around the periphery of the coil. It is due to the fact that the optical fiber coil can be secured sufficiently and reliably to the storage case if there are at least four securing parts. More preferably, at least eight securing parts are formed at substantially identical intervals, since they can endure considerable impacts. Preferably, each resin securing part has a width of about 5 mm. It is due to the fact that no undue pressure will be applied to the optical fiber when a resin is applied thereto in such a width by means of an applicator.

[0095] Preferably, the body portion of the bobbin (see FIG. 3) is coated with a lubricant, such as finely divided powder, before the DCF is wound therearound. It is due to the fact that, when the bobbin is coated with the lubricant, the coefficient of friction between the optical fiber coil and the bobbin surface can be reduced, whereby it becomes easier to pull out the optical fiber coil. For example, in the case of an optical fiber coil manufactured under the condition of the above-mentioned type 1 with no coating applied to the bobbin, it took about 15 minutes for the optical fiber coil to be pulled out from the bobbin. In the case of an optical fiber coil manufactured after the bobbin was coated with talc (see Iwanami Rikagaku Jiten, the fourth edition, p. 239) which was used as a powder inorganic filler, by contrast, it took about 4 minutes for the optical fiber coil to be pulled out from the bobbin. Without being restricted to finely divided powders such as talc, any lubricant may be used as long as it can lower the coefficient of friction between the DCF and the bobbin surface when pulling out the optical fiber coil from the bobbin. For example, it may be a liquid, and it may be applied after coiling as well.

[0096] The tension used when taking up the DCF around the bobbin is preferably as small as possible. It is due to the fact that, when the tension is small, then the friction between the optical fiber coil and the bobbin surface can be lowered, whereby it becomes easier to pull out the optical fiber coil. For example, in the case where an optical fiber coil was manufactured by winding a DCF around a talc-coated bobbin with a take-up tension of 50 gf, it took about 20 minutes for the coil to be pulled out. When an optical fiber coil was manufactured in the same manner except that the take-up tension was changed to 40 gf, by contrast, it took about 4 minutes for the coil to be pulled out. The take-up tension is preferably as low as possible unless the winding state of the DCF becomes disordered. The inventors manufactured various kinds of optical fiber coils with different tensions and, as a result, have found it preferable to set the tension to 40 gf or less in this embodiment.

[0097] The optical fiber coil in a bundle form is not limited to the one produced by a method comprising the steps of winding a DCF around a bobbin and then removing it from the bobbin. During the step of making the optical fiber coil, the DCF may be wound around a member equivalent to a bobbin so as to produce the optical fiber coil, which is then removed from this member.

[0098]FIG. 8 is a schematic view showing a moment at which the optical fiber coil 32 is accommodated in the storage case 40. After the optical fiber coil 32 is secured to the storage case 40 with the aid of the resin (not illustrated), the storage case 40 is closed with a lid 46, whereby the fiber coil 32 is accommodated in the storage case 40 in a state where winding is prevented from becoming disordered. Even in the state closed with the lid 46, interstices exist between the individual turns of the fiber constituting the DCF bundle of the optical fiber coil 32 and between the DCF bundle and the storage case 40, whereby the DCF is kept from being influenced by thermal expansion of its surroundings even when placed in a high-temperature environment.

[0099] Thus accommodated optical fiber coil was subjected to an impact test. The impact test was carried out in compliance with the requirements of IEC68-2-29Eb under the condition with an acceleration of 98 m/sec², an impact application time of 16 msec, and an impact application number of 1000. As indicated by the arrow A in FIG. 8, the direction of impact application was perpendicular to the plane including the wound optical fiber coil. Even after this impact test, no disorder in winding occurred in the optical fiber coil. No increase in transmission loss was found when the transmission loss value was measured at a wavelength of 1.55 μm, and no increase in loss other than that inherent in the optical fiber was found when measured after the temperature was raised to 70° C.

[0100] Thus, as the optical fiber coil 32 in a bundle state is secured to the storage case 40 with the resin 42, it can be accommodated in the storage case in the state where distortions in winding of the optical fiber coil are substantially released without generating disorders in winding. As a consequence, the optical fiber coil is prevented from being broken or changing its characteristics due to vibrations, impacts, and the like, and it is possible to obtain a small-sized dispersion compensator whose transmission loss value and temperature dependence of transmission loss are reduced.

[0101] Second Embodiment

[0102] The dispersion compensator in accordance with a second embodiment will now be explained. FIG. 9 is a side view showing the dispersion compensator in accordance with the second embodiment. In this embodiment, an optical fiber coil 48 is secured to a bobbin 2 in a state wound around the bobbin 2. Namely, with the aid of a resin applied to four resin application portions 52 located at a flange 26 of the bobbin 2, the optical fiber coil 48 is secured to the flange 26, i.e., bobbin 2, whereas the DCF 50 is drawn out from both ends of the optical fiber coil 48. Though the portions coated with the resin are actually hidden inside the flange 26, they are illustrated explicitly for the sake of explanation.

[0103]FIG. 10 is a vertical sectional view of this dispersion compensator. A resin 54 is applied so as to secure the optical fiber coil 48 to two sheets of flanges 26 on the inside thereof. As the resin 54, the one identical to that of the first embodiment can be used. Thus, the optical fiber coil 48 is secured at a plurality of positions from therearound. Preferably, at least four securing parts are formed by the resin at substantially identical intervals. It is due to the fact that the optical fiber coil 48 can be secured sufficiently to the flange 26, i.e., bobbin 2, if there are at least four securing parts. More preferably, at least eight securing parts are formed at substantially identical intervals, since they can endure considerable impacts. Preferably, the resin 54 has a width of about 5 mm. It is due to the fact that no undue pressure will be applied to the fiber constituting the coil when a resin is applied thereto in such a width by means of an applicator.

[0104] The configuration of the bobbin 2 will now be explained with reference to FIGS. 11A to 11C and 12A to 12C. FIG. 11A is an exploded view of the bobbin 2, FIG. 11B is a side view of a wedge member 24 thereof, and FIG. 11C is a sectional view of the bobbin 2 taken along a plane including the center axis thereof. FIG. 12A is a top plan view of a support member 28, FIG. 12B is a lateral sectional view of the support member 28, and FIG. 12C is an enlarged view of a groove 36.

[0105] As shown in FIG. 11A, the bobbin 2 comprises a cylindrical core member 22, two flanges 26, a support member 28, and a plurality (10 pieces in FIG. 9) of wedge members 24. Each flange 26 is a thin disc having an opening at the center part, into which the core member 22 is inserted. The support member 28 has an opening at the center part, into which the core member 22 is inserted. As shown in FIG. 11B, each wedge member 24 has a sector form which is obtained when a disc is divided into 10 equal parts by lines intersecting at the center thereof. The center part of the wedge member 24 is provided with a protrusion 30 on each side, whereas the circumferential part has a thick portion having a dimension identical to the winding width of the coil.

[0106] When these members are assembled together, then, as shown in FIG. 11C, the two flanges 26 hold the support member 28 from both sides, and these members are arranged concentrically. When the core member 22 is inserted into the opening located at the center part of thus arranged members, the flanges 26 and the support member 28 are secured to each other thereby. The wedge members 24 are held between the support member 28 and the flanges 26 on both sides thereof so as to be secured, while coming into contact with the core member 22 at their tips.

[0107] As shown in FIGS. 12A and 12B, the support member has a structure in which two discs are bonded to each other with connection walls 34 each extending from the circumferential side to the edge of the opening. In order to insert the wedge members 24 therebetween, 10 pieces of the connection walls 34 are disposed in conformity to the form of the wedge part. The center part between each pair of the neighboring connection walls 34 is formed with a groove 36 extending from the peripheral side toward the center. As shown in FIG. 12C, the groove 36 is made of a rectangular parallelepiped depression. Disposed within the groove 36 is a helical spring 38 whose one end is secured to the peripheral side of the support member 28. The helical spring is employed here since it has a simple structure and can be made with a small size. In place of the helical spring, a variety of elastic bodies can be employed as well.

[0108] With reference to FIGS. 13A to 13D, a mechanism for reducing the body diameter of the bobbin 2 after taking up a DCF will now be explained. In FIGS. 13A to 13D, in order for the compressing mechanism to be easily discernible, the flanges 26 are omitted, and the wedge members 24 hidden inside the support member 28 are indicated by broken lines.

[0109]FIGS. 13A and 13C are a side view and a top plan view each showing the bobbin before its body diameter is reduced. The wedge members 24 are inserted between the connection walls 34 of the support member 28 with their wedge parts directed to the center and come into contact with the core member 22 at their tips. The protrusions 30 on both sides fit into their corresponding grooves 36 while compressing the respective helical springs 38. As a consequence, each wedge member 24 receives the force directed toward the center from its corresponding helical spring 38, so as to be secured by the core member 22 having a diameter of 10 mm. Hence, the arcs of 10 wedge members 24 form substantially a cylindrical body portion of the bobbin. Thus formed cylindrical surface is used for winding the DCF (not illustrated). Here, the body diameter g is 100 mm, and the winding width k is 18 mm.

[0110]FIGS. 13B and 13D are a side view and a top plan view each showing the bobbin after its body diameter is reduced. When the core member 22 is pulled out from the bobbin 2, then the wedge members 24 move toward the center due to the forces of the helical springs 38 so that their wedge parts are put together, whereby a compressed cylindrical body portion of the bobbin is formed. After this compression, the body diameter g is 90 mm.

[0111] The compressible bobbin structure is not limited to that explained above. Any specific mechanism can be used as long as the body diameter in the state accommodated in the dispersion compensator is substantially reduced as compared with that at the time when the DCF is being wound therearound. The number of the wedge members 24, the structure of the groove 36, and so forth are given by way of illustration only, and other configurations may be employed as long as their functions can be exhibited. Here, “substantially reduced” refers to such a compression that distortions in winding are released, whereby the transmission loss and the temperature dependence of transmission loss can be reduced.

[0112] As can be seen from the foregoing explanations, the degree of reduction in body diameter can be modified by the diameter of the core member 22. Preferably, the ratio of reduction is a few % with respect to the body diameter. Such a degree of reduction can substantially release distortions in winding, whereby lateral pressures can be alleviated. For fully alleviating the lateral pressures, it is more preferable that the ratio of reduction be 5% to 6%. Here, it is unnecessary for the degree of reduction to exceed 10% at most.

[0113] The DCF having the characteristics mentioned above and a length of 10 km was wound around the above-mentioned compressible bobbin under the condition with a winding pitch of 0.4 mm and a take-up tension of 40 gf, so as to produce an optical fiber coil. The transmission loss value before reducing the body diameter of the bobbin was measured and found to be 1.7 dB at a wavelength of 1.55 μm. Subsequently, after the body diameter was reduced to 90 mm, the optical fiber coil was loosened without changing its outside diameter of winding and was swung so as to homogenize the winding state of the whole optical fiber or its state of irregularity, and then the transmission loss value was measured, whereby it was found that the increase in transmission loss value generated upon coiling disappeared. When the transmission loss value of this coil was measured after the coil was heated to 70° C., the increase in loss compared with that at 20° C. was 0.06 dB, indicating that no increase in loss occurred other than that inherent in the optical fiber.

[0114] When the bobbin is thus compressed after a plurality of turns of DCF are wound therearound to produce an optical fiber coil, then the optical fiber coil is placed in a state substantially released from distortions in winding. As a consequence, the lateral pressures generated due to a multilayer of winding of the DCF are alleviated, and the optical fiber coil is kept from being influenced by thermal expansions of the bobbin even when placed in a high-temperature environment, whereby the transmission loss of the optical fiber coil and its temperature dependence of the transmission loss can be reduced. Namely, the optical fiber coil can be accommodated in the bobbin in a state where the wavelength-dispersion-compensating optical fiber is substantially released from distortions in winding.

[0115] Then, the optical fiber coil in a state where the coil was secured to the bobbin at several positions as shown in FIGS. 9 and 10 was subjected to an impact test under the same condition as that of the first embodiment. As indicated by the arrow A in FIG. 10, the direction of impact application was perpendicular to the plane including the optical fiber coil 48. Even after this impact test, no disorder in winding occurred in the optical fiber coil. Also, no increase in transmission loss was found when the transmission loss value was measured at a wavelength of 1.55 μm, and no increase in loss other than that inherent in the optical fiber was found when measured after the temperature was raised to 70° C.

[0116] As explained in the foregoing, when the body diameter of the bobbin is reduced after the DCF is wound therearound, and the optical fiber coil 48 is secured to the flanges 26 of the bobbin 2 with the resin 52, then the bobbin 22, which also acts as a storage case, can accommodate the optical fiber coil 48 in a state where the optical fiber coil is substantially released from distortions in winding loss without causing the winding to become disordered. As a consequence, the optical fiber coil is prevented from being broken or changing its characteristics due to vibrations, impacts, and the like, and it is possible to obtain a small-sized dispersion compensator whose transmission loss value and temperature dependence of transmission loss are reduced.

[0117] Third Embodiment

[0118] The dispersion compensator in accordance with a third embodiment will now be explained. In the dispersion compensator in accordance with the third embodiment, an optical fiber coil in a bundle form disposed within a storage case is secured by means of a cushion material acting as a coil-tidying member. FIG. 14 is an exploded perspective view of the third embodiment, whereas FIG. 15 is a perspective view of its cushion material 60.

[0119] As shown in FIG. 15, the cushion material 60 for accommodating an optical fiber coil 32 has an outer shape in conformity to the inner form of a storage case 62, whereas its upper face is formed with an optical fiber coil storage section 61 formed into a groove in conformity to the form of the optical fiber coil 32. Namely, the optical fiber coil storage section 61 comprises an annular groove area 63 a shaped in conformity to the optical fiber coil 32, and two linear groove areas 63 b which are portions for drawing out the optical fiber coil 32. Preferably, the depth of the groove in the optical fiber coil storage section 61 is such that the optical fiber coil 32 is accommodated therein while leaving a margin for disposing a lid 66 of cushion material thereon. In this manner, the cushion materials 60 and 66 can surround the optical fiber coil 32. Though the cross section of the groove is U-shaped here, without being restricted thereto, it may be V-shaped or rectangular as well.

[0120] Referring to FIG. 14, an assembling method in accordance with this embodiment will now be explained. The cushion material 60 is put into the storage case 62, and the optical fiber coil 32 is accommodated in the annular groove area 63 a of the optical fiber storage section 63 of the cushion material 60. Then, both ends of the DCF are drawn out from the optical fiber coil 32, and are connected to their corresponding pigtail fibers 45 at their respective fusion-splicing parts 44. Each fusion-splicing part 44 is accommodated in the linear groove area 63 b of the optical fiber storage section 63, whereas the free end of each pigtail fiber 45 is drawn out of the storage case 62. Thereafter, the lid 66 of cushion material is placed so as to mate with the optical fiber storage section 63, and the storage case 62 is closed with a lid 64. As a result, the optical fiber coil 32 is secured, whereby the dispersion compensator in accordance with this embodiment is accomplished.

[0121] Here, the lid 66 of cushion material preferably has a form covering the opening area of the optical fiber storage section 63, and it is more preferable that this form be substantially identical to the opening form of the optical fiber storage section 63. It is due to the fact that the optical fiber coil 32 can be covered with the cushion materials 63 and 66 thereby.

[0122] It is required for the cushion materials 60 and 66 to be processed easily, be soft enough so as not to damage the optical fiber, and have such a low Young's modulus that they can deform elastically so as to surround the optical fiber with a slight pressure when accommodated. As such a cushion material, flexible polyurethane and polyethylene foam are preferable. In particular, polyethylene foam is preferable in practice due to its easiness in processing. However, any material can be used as long as the above-mentioned requirements are satisfied.

[0123] Also, the form of the cushion material 60 is not limited to that shown in FIGS. 14 and 15. For example, a cushion member 60 b having a form shown in FIG. 16 may be used as well. This cushion material 60 b has a form in which the cushion material 60 shown in FIG. 15 is partly cut off. As a result, the annular groove area 63 a of the optical fiber storage section 63 has a divided form. This configuration is advantageous in that, even when the outer form of the cushion material 60 b is slightly greater than the inside of the storage case 62, the cushion material 60 b can easily be accommodated in the storage case 62, and that the cushion material 60 b can reliably be secured into the storage case 62 due to its elastic deformation.

[0124] An impact test was carried out under the same condition as that of the first and second embodiments while using polyethylene foam (product code: PE Light B4, manufactured by INOAC Corporation; having a density of 0.027 g/cm³) as the cushion members 60 and 66, results of which will now be reported. The direction of impact application was indicated by the arrow A in FIG. 14, i.e., perpendicular to the plane including the optical fiber coil 32. Even after this impact test, no disorder in winding occurred in the optical fiber coil. Also, no increase in transmission loss was found when the transmission loss value was measured at a wavelength of 1.55 μm, and no increase in loss other than that inherent in the optical fiber was found when measured after the temperature was raised to 70° C.

[0125] As explained in the foregoing, when the optical fiber coil 32 is accommodated in the optical fiber storage section 63 formed by recessing the cushion member 60, then the optical fiber coil 32 can be accommodated in the storage case 62 in a bundle state substantially released from distortions in winding, without causing the winding to become disordered. As a consequence, the optical fiber coil is prevented from being broken or changing its characteristics due to vibrations, impacts, and the like, and it is possible to obtain a small-sized dispersion compensator whose transmission loss value and temperature dependence of transmission loss are reduced.

[0126] Fourth Embodiment

[0127] The dispersion compensator in accordance with a fourth embodiment will now be explained. In the dispersion compensator in accordance with the fourth embodiment, as shown in FIG. 17, an optical fiber coil 48 wound around a bobbin 2 is secured to the bobbin 2, also acting as a storage section, with the aid of a cushion material 68 which is a coil-tidying member.

[0128] As the cushion material 68 is held between the flanges 26 on both sides of the bobbin 2 and is wound around the whole outer periphery of the flanges 26, the optical fiber coil 48 is secured to the flanges 26. The cushion material 68 is secured to the flanges 26 with a resin. Preferably, the resin is applied so as not to adhere to the optical fiber coil 48. The size of the cushion material 68 varies depending on the size of the flanges 26 used, the number of turns of the DCF, and the like. In the case where an optical fiber coil similar to that of the second embodiment is used, the cushion material 68 is preferably formed into a rectangular parallelepiped having a thickness of 3 mm, a width of 18 mm in conformity to the winding width of the coil, and a length of about 628 mm in conformity to the outer peripheral length of the flanges 26.

[0129] The form of the cushion member is not limited to that shown in FIG. 17. As shown in FIG. 18, a plurality of thin rectangular parallelepiped cushion materials 72 may be disposed at a plurality of positions, so as to secure the optical fiber coil 48 to the flanges 26. Also, the cushion material 68 is preferably provided with outlets 70 for the DCF wound into a coil form.

[0130] Though the cushion material has characteristics identical to those of the third embodiment, a silicone resin is preferably used as its resin. However, any resin may be used as long as it does not peel off when a heat history is applied thereto within the range of about 0° C. to about 70° C., which is assumed for an environment of use.

[0131] Thus accommodated optical fiber coil was subjected to an impact test carried out under the same condition as that of the third embodiment. The direction of impact application was indicated by the arrow A in FIG. 17, i.e., perpendicular to the plane including the optical fiber coil 48. Even after this impact test, no disorder in winding occurred in the optical fiber coil 48. Also, no increase in transmission loss was found when the transmission loss value was measured at a wavelength of 1.55 μm, and no increase in loss other than that inherent in the optical fiber was found when measured after the temperature was raised to 70° C.

[0132] As explained in the foregoing, when the optical fiber coil 48 wound around the bobbin in a state substantially released from distortions in winding is secured with the cushion material 68, then the optical fiber coil 48 can reliably be secured to the bobbin without causing the winding to become disordered. As a consequence, the optical fiber coil is prevented from being broken or changing its characteristics due to vibrations, impacts, and the like, and it is possible to obtain a small-sized dispersion compensator whose transmission loss value and temperature dependence of transmission loss are reduced.

[0133] Fifth Embodiment

[0134] The dispersion compensator in accordance with a fifth embodiment will now be explained. In the dispersion compensator in accordance with the fifth embodiment, the storage case in the first embodiment is further filled with a cushioning filler so as to secure the optical fiber coil.

[0135]FIG. 19 shows a sectional configurational view of this embodiment. As with the first embodiment, an optical fiber coil 32 in a state substantially released from distortions in winding is accommodated in a storage case 80 having a rectangular or circular bottom face. The storage case 80 is filled with a filler 84 which surrounds the optical fiber coil 32. Here, the filler 84 preferably fills in between turns of the optical fiber constituting the optical fiber coil 32 as well. The storage case 80 is mounted with a lid 82 so as to be sealed therewith.

[0136] Preferable as the filler 84 are thermosetting or UV-curable silicone resins having a Young's modulus of 0.05 kg/mm² or less; jelly-like admixtures having a high viscosity in which a rubber such as that of butadiene or silicon is swollen with a solvent, such as silicone or naphthene, and other resins or the like are optionally added thereto; and the like. As such a highly flexible material with a high viscosity is used as a filler, the optical fiber constituting the optical fiber coil 32 can reliably be secured without applying thereto an excess force which may impart distortions in winding to the optical fiber. Also, as the filler fills in between turns of the fiber, the pressures applied to the individual turns of the fiber are homogenized, whereby the microbend loss occurring due to irregular lateral pressures can be suppressed. Consequently, a dispersion compensator using a small-diameter fiber (having a glass diameter of 100 μm or less and a coating thickness of 140 μm or less) or a fiber having a low Δn, which has conventionally been hard to use due to its vulnerability to distortions in winding, can easily be made. When a small-diameter fiber is used, the optical fiber coil can be made compact, whereby the module can have a much smaller size. When a fiber having a low Δn is used, then the effective core cross-sectional area A_(eff) can be reduced, so as to suppress the nonlinear characteristic.

[0137] The form of the storage case 80 is not limited to that shown in FIG. 19. It may have an annular configuration as with the storage case 80 a shown in FIG. 20. Though FIGS. 19 and 20 depict the optical fiber coil 32 in a form in contact with the bottom face of the storage case 80 or 80 a, the optical fiber coil 32 may be accommodated in the storage case 80 or 80 a after a small amount of the filler is injected therein, before the rest of the filler is injected and cured therein, for example. This procedure is more preferable in that the pressure applied to the optical fiber coil 32 from the bottom face of the storage case 80 or 80 a can be alleviated.

[0138] The optical fiber coil 32 may also be accommodated in the storage case 80 or 80 a after being produced in a manner similar to that of the first embodiment, or a DCF may be dropped into the storage case 80 or 80 a and formed into a coil therein.

[0139] After the optical fiber coil 32 is formed in this way, a resin (filler) is applied onto the periphery of the coil and then cured. This curing can be thermal curing or ultraviolet curing, depending upon the properties of the resin. In an example of the thermal curing, the resin is cured by heating at 50° C. for 24 hours.

[0140] During the application operation of the filler (resin), the surface tension of the resin is preferably not more than 40 dyn/cm², in order to prevent creation of bubbles etc. between fiber windings in the coil form. In order to allow the resin to fully penetrate into the spaces between the fiber windings, it is also preferable that the viscosity of the resin under the application be kept not more than 10000 cs. Further, it is preferable that during the curing of the resin the viscosity of the resin be kept not more than 10000 cs for two or more hours from the start of curing. This is for the purposes of allowing the resin to fully penetrate into the spaces between the fiber windings and preventing occurrence of unwanted microbend during the curing.

[0141] As explained in the foregoing, when surrounded with the filler 84, the optical fiber coil 32 can be accommodated in the storage case 80 in a state where distortions in winding are substantially released, without causing the winding to become disordered. As a consequence, the optical fiber coil is prevented from being broken or changing its characteristics due to vibrations, impacts, and the like, and it is possible to obtain a small-sized dispersion compensator whose transmission loss value and temperature dependence of transmission loss are reduced.

[0142] Sixth Embodiment

[0143] The dispersion compensator in accordance with a sixth embodiment will now be explained. This embodiment differs from the fourth embodiment, in which the optical fiber coil is secured to the bobbin with the cushion material wound around the outer peripheral portion, in that the optical fiber coil is secured with a filler filling in between turns of the fiber constituting the optical fiber coil and between the fiber and the bobbin.

[0144]FIG. 21 is an enlarged sectional view of the sixth embodiment. A filler 84 fills in between turns of the optical fiber of the optical fiber coil 48 wound around the body portion 24 of the bobbin 2 and between the individual turns of the optical fiber and the outer peripheral surface of the body portion 24 of the bobbin 2 or the wall face of each flange 26. As the filler 84, a material identical to that of the filler of the fifth embodiment can be used.

[0145] As such a highly flexible material with a high viscosity is used as a filler, the optical fiber constituting the optical fiber coil 48 can reliably be secured without applying thereto an excess force which may impart distortions in winding to the optical fiber. Also, as the filler fills in between turns of the fiber, the pressures applied to the individual turns of the fiber are homogenized, whereby the microbend loss occurring due to irregular lateral pressures can be suppressed. Consequently, a dispersion compensator using a small-diameter fiber or a fiber having a low Δn, which has conventionally been hard to use due to its vulnerability to distortions in bending, can easily be made.

[0146] Also, in this embodiment, even in the case where aluminum or the like having a relatively high coefficient of thermal expansion is used as the bobbin 2, if the outer periphery of the body portion 24 of the bobbin 2 expands, then the filler 84 absorbs the stress caused by the expansion, so as to keep the stress from being transmitted to the optical fiber coil 48, whereby the optical fiber coil 48 can be prevented from being distorted upon thermal expansion. As a consequence, reliability improves, and it is unnecessary for the bobbin 2 to use an expensive material having a low thermal expansion, whereby the cost can be cut down.

[0147] Methods of making the dispersion compensator in accordance with the sixth embodiment will now be explained. FIGS. 22 and 23 are views for explaining these methods.

[0148] In the method shown in FIG. 22, a DCF 96 wound around a feed bobbin 90 is taken up around the bobbin 2 by way of capstan rollers 91, 92. Here, the outer periphery of the DCF 96 is substantially uniformly coated with a filler 84 in a liquid form by a coating means 94 disposed in front of the bobbin 2. As a result, the interstices in the DCF 96 wound around the bobbin 2 are substantially uniformly filled with the filler 84.

[0149] In the method shown in FIG. 23, the coating means 94 is disposed on the side of the bobbin 2, so as to apply the filler 84 to the outer peripheral surface of the body portion 24 of the bobbin 2, i.e., the surface around which the DCF 96 is being wound or has already been wound. As a consequence, the interstices of the DCF 96 being wound can reliably be coated with the filler 84.

[0150] As explained in the foregoing, when the filler 84 is placed between the turns of the optical fiber coil 48 wound around the bobbin 2 in a state substantially released from distortions in winding, then the optical fiber coil 48 can reliably be secured to the bobbin without causing the winding to become disordered. As a consequence, the optical fiber coil is prevented from being broken or changing its characteristics due to vibrations, impacts, and the like, and it is possible to obtain a small-sized dispersion compensator whose transmission loss value and temperature dependence of transmission loss are reduced.

[0151] Seventh Embodiment

[0152] In this embodiment, as shown in FIG. 24, a DCF is wound around a take-up bobbin 2, so as to form an optical fiber coil 32.

[0153] This embodiment is characterized in that, during or after being taken up, the optical fiber coil 32 is adjusted into a state substantially released from the above-mentioned distortions in winding. Various techniques can be employed for adjusting the optical fiber coil into this state, which will be explained in the following with reference to Examples A to L by way of illustration.

EXAMPLE A

[0154] Around a bobbin 2 made of aluminum in which g=120 mm and k=45 mm, about 90 plies of a DCF having a length of 9 km, the form shown in FIG. 1, and the refractive index profile shown in FIG. 2 were wound with a winding pitch of 0.28 mm at each of two kinds of take-up tensions of 50 gf and 70 gf, whereby two kinds of dispersion compensators were produced. This DCF has the specific characteristics shown in the following Table. Characteristics Values Δ+  2.5% Δ− −0.5% a 2.54 μm b 7.26 μm c  100 μm d   20 μm e   20 μm f  180 μm Young's modulus of primary 0.06 kgf/mm² coating layer 13 Young's modulus of secondary 65 kgf/mm² coating layer 15

[0155]FIG. 25 is a graph plotting thus obtained two kinds of dispersion compensators in terms of take-up tension and transmission loss value at a wavelength of 1.55 μm which are indicated by the abscissa and the ordinate, respectively. The broken line indicates the transmission loss value (0.53 dB/km) of the DCF at a wavelength of 1.55 μm in the state wound around a plastic bobbin having a body diameter of 280 mm, for comparison. In Example A, the transmission loss value was 3.01 dB/km and 1.98 dB/km when the take-up tension was 70 gf and 50 gf, respectively.

EXAMPLE B

[0156] Around a bobbin 2 made of aluminum in which g=60 mm and k=45 mm, about 190 plies of a DCF, identical to that of Example A, having a length of 13 km were wound with a winding pitch of 0.28 mm at each of three kinds of take-up tensions of 20 gf, 25 gf, and 50 gf, whereby three kinds of dispersion compensators were produced.

[0157]FIG. 26 is a graph plotting thus obtained three kinds of dispersion compensators in terms of take-up tension and transmission loss value at a wavelength of 1.55 μm which are indicated by the abscissa and the ordinate, respectively. The broken line is a transmission loss value shown for comparison as with FIG. 25. The transmission value was 2.54 dB/km and 1.05 dB/km when the take-up tension was 50 gf and 25 gf, respectively, thereby becoming smaller as the take-up tension was lower. At the take-up tension of 20 gf, however, the transmission loss value increased to 2.95 dB/km. It is assumed to be because the take-up tension was so low that the winding of the fiber loosened, thereby generating distortions in winding.

EXAMPLE C

[0158] Around a bobbin 2 made of aluminum in which g=100 mm and k=18 mm, about 210 plies of a DCF having a length of 8 km and the characteristics shown in the following Table (see FIGS. 1 and 2 for its form and refractive index profile, respectively) were wound with a winding pitch of 0.40 mm at each of four kinds of take-up tensions of 20 gf, 30 gf, 40 gf, and 50 gf, whereby four kinds of dispersion compensators were produced. This DCF has the specific characteristics shown in the following Table. Characteristics Value Δ+  2.1% Δ−  −0.35% a 2.65 μm b 7.58 μm c  110 μm d   20 μm e   15 μm f  180 μm Young's modulus of 0.03 kgf/mm² primary coating layer 13 Young's modulus of  100 kgf/mm² secondary coating layer 15

[0159]FIG. 27 is a graph plotting thus obtained four kinds of dispersion compensators in terms of take-up tension and transmission loss value at a wavelength of 1.55 μm which are indicated by the abscissa and the ordinate, respectively. The broken line also indicates take-up tension and transmission loss value by the abscissa and the ordinate, respectively. The broken line indicates the transmission loss value (0.45 dB/km) at a wavelength of 1.55 μm before the coil is taken up. The transmission loss value was 0.50 dB/km, 0.48 dB/km, and 0.46 dB/km when the take-up tension was 50 gf, 40 gf, and 30 gf, respectively, thereby becoming smaller as the take-up tension was lower. At the take-up tension of 20 gf, however, the winding of the fiber loosened, whereby the transmission loss value increased and exceeded 1.0 dB/km.

[0160] As can be seen from the relationship between transmission loss value and take-up tension in Examples A, B, and C, when the take-up tension for winding the optical fiber is lowered, then the increase in transmission loss value caused by the winding can be reduced. It has been confirmed that this take-up tension is preferably 50 gf or less, but more preferably at least 25 gf at which the winding of the fiber would not loosen. When the optical fiber coil is produced within such a range of take-up tension, then the lateral pressures occurring upon winding can be suppressed, whereby the increase in transmission loss value can be lowered. More preferably, the take-up tension is at least 30 gf but not greater than 40 gf. When the winding is carried out within such a tension range, the increase in transmission loss value can further be lowered.

EXAMPLE D

[0161] Around a bobbin 2 made of aluminum in which g=80 mm and k=45 mm, about 190 plies of a DCF having a length of 14 km and the characteristics shown in the following Table (see FIGS. 1 and 2 for its form and refractive index profile, respectively) were wound at a take-up tension of 35 gf with each of four kinds of winding pitches of 0.2 mm, 0.4 mm, 0.6 mm, and 1.0 mm, whereby four kinds of dispersion compensators were produced. This DCF has the specific characteristics shown in the following Table. Characteristics Value Δ+  2.1% Δ−  −0.35% a 2.67 μm b 7.63 μm c   98 μm d   20 μm e   30 μm f  198 μm Young's modulus of 0.12 kgf/mm² primary coating layer 13 Young's modulus of   50 kgf/mm² secondary coating layer 15

[0162]FIG. 28 is a graph plotting thus obtained four kinds of dispersion compensators (with black circles) in terms of winding pitch and PMD which are indicated by the abscissa and the ordinate, respectively. While PMD was 1.68 ps when the winding pitch was 0.2 mm, it became 1.12 ps, 0.94 ps, and 0.90 ps at the winding pitches of 0.4 mm, 0.6 mm, and 1.0 mm, respectively. Hence, it has been confirmed that PMD can be lowered when the winding pitch is made greater as compared with the diameter of the DCF to be wound. Preferably, the ratio of the winding pitch to the DCF diameter is set to a value of 2 or greater, since PMD can considerably be lowered thereby. More preferably, this ratio is set to a value of 3 or greater, since the dependence on the winding pitch can be made very small thereby.

[0163] If the winding pitch is thus controlled at the time when the DCF is wound into a small-sized coil, then mode coupling occurs between orthogonal polarization modes, whereby PMD can be reduced.

[0164] On the other hand, the transmission loss of thus produced optical fiber coil at a wavelength of 1.55 μm was 0.46 dB/km when the winding pitch was 0.6 mm. This coil was heated to 80° C. at a temperature changing rate of 0.5° C./min, held for 1 hour at this temperature, and then returned to room temperature, at which the transmission loss value was measured again and found to be 0.44 dB/km, which was identical to the transmission loss value before coiling. The PMD at this time was 0.79 cps as indicated by the black triangle in the graph.

[0165] It has thus been confirmed that, when the winding pitch is controlled while the DCF is being wound into a small-sized coil, and the DCF is subjected to a heat treatment after being wound, then stress is alleviated, whereby the increase in transmission loss value can be reduced. Further, it has been confirmed that the increase in PMD can be reduced as well.

EXAMPLE E

[0166] Around a bobbin 2 made of aluminum in which g=100 mm and k=35 mm, about 120 plies of a DCF having a length of 6 km and the characteristics shown in the following Table (see FIGS. 1 and 2 for its form and refractive index profile, respectively) were wound at a take-up tension of 40 gf with a winding pitch of 0.8 mm, and then the bobbin was vibrated for 30 minutes, so as to substantially release distortions in winding, whereby a dispersion compensator was produced. This DCF has the specific characteristics shown in the following Table. Characteristics Values Δ+  2.5% Δ− −0.5% a 2.57 μm b 7.34 μm c  115 μm d   0 μm e   70 μm f  225 μm Young's modulus of secondary 40 kgf/mm² coating layer 15

[0167] A vibration tester was used for applying the vibration to the bobbin, such that the displacement of the bobbin caused by the vibration over time drew a sinusoidal wave. The vibration frequency was 50 Hz, the maximum acceleration was 10 m/sec², and the vibration was generated in Z directions in FIG. 24. The transmission loss value of the optical fiber at a wavelength of 1.55 μm was 0.44 dB/km before coiling and increased to 0.45 dB/km after coiling. After the vibration was added thereto, the transmission loss value became 0.44 dB/km, i.e., returned to the value obtained before coiling.

[0168] It has thus been confirmed that, when a vibration is applied to the DCF, its radius of curvature can be increased or lateral pressures can be alleviated, whereby the increase in transmission loss value can be reduced. It is not necessary to apply the vibration continuously, as long as the total time of vibration is at least 30 minutes. For example, the vibration may be applied intermittently over a certain period of time as well.

[0169] The direction of vibration of the bobbin may not only be parallel to the flange surfaces of the bobbin (Z directions in FIG. 24) but also be substantially perpendicular to the flange surfaces of the bobbin, or may form a predetermined angle with these directions. When applying the vibration to the bobbin, it is preferably provided with means for preventing the winding of the optical fiber coil from becoming disordered, such as that shown in the above-mentioned third embodiment.

EXAMPLE F

[0170] Around a bobbin 2 made of aluminum in which g=120 mm and k=45 mm, about 90 plies of a DCF, identical to that of Example A, having a length of 9 km were wound with a winding pitch of 0.28 mm at a take-up tension of 50 gf, and then a predetermined heat treatment was carried out, whereby a dispersion compensator was produced. FIG. 29 shows the heat cycle of this heat treatment, in which the initial temperature was 20° C., the range of temperature change was from 80° C. to −40° C., the temperature changing rate was 1° C./min, two cycles of heating and cooling were carried out with 1 hour of retention at each of 80° C. after heating and −40° C. after cooling, and retention at 20° C. was effected for 1 hour before changing temperature and for 2 hours before the termination.

[0171]FIG. 30 shows the amount of change in transmission loss at a wavelength of 1.55 μm over time. Here, the transmission loss value before starting the heat treatment was 1.98 dB/km, whereas that in the state wound around a plastic bobbin having a body diameter of 280 mm was 0.53 dB/km. The change in transmission loss value caused by the heat treatment was about −1.4 dB/km, whereby the increase generated upon coiling disappeared. Such a heat cycle yielded a temperature change of ±60° C. with respect to the initial temperature as well. As can be seen from FIG. 30, after this heat cycle was carried out twice, the transmission loss value was substantially stabilized. It has thus been confirmed that the heat treatment can alleviate transmission loss.

EXAMPLE G

[0172] Around a bobbin 2 made of aluminum in which g=60 mm and k=45 mm, about 90 plies of a DCF, identical to that of Example A, having a length of 9 km were wound with a winding pitch of 0.28 mm at a take-up tension of 25 gf, and then a predetermined heat treatment was carried out, whereby a dispersion compensator was produced. FIG. 31 shows the heat cycle of this heat treatment, which is the same as that shown in FIG. 29 except that the order of the heating and cooling is reversed.

[0173] While the transmission loss value at a wavelength of 1.55 μm was 1.05 dB/km before the heat treatment, it became 0.53 dB/km after the heat treatment, whereby the increase generated upon coiling disappeared.

[0174] As can be seen from Examples F and G, in the case where the coiled DCF is subjected to a heat treatment, the coating shrinks when cooled to a temperature lower than that at the time of winding, whereas the stress is alleviated by heat when heated to a temperature higher than that at the time of winding, so that the radius of curvature of the wound DCF can be enhanced, whereby the increase in transmission loss value can be reduced. It has also been confirmed that, when the heat cycle is repeated a plurality of times, then the transmission loss value is stabilized with respect to core noncircularity of the DCF is at most about 1%. Even in the case of such DCF with the core noncircularity of about 1%, mode coupling is promoted between polarization modes by giving the twist of 90° per meter, whereby the polarization dispersion of not more than 0.5 ps/rkm can be realized. 

39. (New) An optical fiber coil comprising: a first optical fiber being wound so as to have a coil part with a coil shape and having a bundle form in said coil part; and a resin inserted between parts of said first optical fiber in said bundle form, which are adjacent to among them, wherein a quarter cone penetration of said resin, which is defined in JIS K 2220, is between 5 and 200 both inclusive at 25° C.
 40. (New) A optical fiber coil comprising: a first optical fiber being wound so as to have a coil part with a coil shape and having a bundle form in said coil part; and a resin inserted between parts of said first optical fiber in said bundle form, which are adjacent to among them, wherein a quarter cone penetration of said resin, which is defined in JIS K 2220, is not less than 5 at −40° C. and not more than 200 at 100° C.
 41. (New) The optical fiber coil according to claim 39, wherein said first optical fiber coil has no bobbin.
 42. (New) The optical component according to claim 39, further comprising a housing case, wherein said optical fiber coil is placed in said housing case and said resin is placed in said housing case.
 43. (New) The optical fiber coil according to claim 39, wherein said first optical fiber is released in the bundle form.
 44. (New) The optical fiber coil according to claim 42, wherein said optical fiber coil is formed by falling said wounded optical fiber into said housing.
 45. (New) The optical fiber coil according to claim 39, wherein at wavelengths in a used wavelength region, at least one of chromatic dispersion coefficient and chromatic dispersion slope of said first optical fiber has a negative value.
 46. (New) The optical fiber coil according to claim 45, wherein said used wavelength region is not less than the wavelength of 1.50 μm.
 47. (New) The optical fiber coil according to claim 39, wherein said first optical fiber has a coated layer, and said coating layer comprises a thin film having at least one function of improving of the strength of the first optical fiber and a function of restraining hydrogen from intruding into a glass of said first optical fiber.
 48. (New) The optical fiber coil according to claim 47, wherein said thin film is made of a carbon.
 49. (New) The optical fiber coil according to claim 47, wherein said first optical fiber has a covering layer on said thin film.
 50. (New) The optical fiber coil according to claim 39, further comprising; a second optical fiber having a connection portion through which said second optical fiber is connected. wherein said second optical fiber is different from said first optical fiber.
 51. (New) The optical fiber coil according to claim 50, said first optical fiber and said second optical fiber are fusion-spliced to said second optical fiber at said connecting portion to each other.
 52. (New) The optical fiber coil according to claim 50, wherein bending loss of said first optical fiber is smaller than that of said second optical fiber.
 53. (New) The optical fiber coil according to claim 50, wherein in said second optical fiber is designed for the 1.55 μm band operation and its core does not contain GeO₂.
 54. (New) The optical fiber coil according to claim 50, wherein said second optical fiber is a Hytrel-coated fiber.
 55. (New) The optical fiber coil according to claim 39, wherein said optical fiber coil is comprised of two optical fiber coils, and wherein first ends thereof are spliced to each other and second ends thereof not spliced are drawn from a common end side of the optical fiber coils.
 56. (New) The optical fiber coil according to claim 39, wherein an amount of hydrogen evolving from said resin is not more than 0.001 ml/g during a thermal treatment of 60° C.×24 hours.
 57. (New) The optical fiber coil according to claim 39, wherein a refractive index of said resin is larger than that of a cladding of said first optical fiber.
 58. (New) The optical fiber coil according to claim 39, wherein said resin contains a hydrogen absorber.
 59. (New) The optical fiber coil according to claim 39, wherein at wavelengths in a used wavelength region, said first optical fiber has bending loss of not less than 1 dB/m when bent in the diameter of 20 mm.
 60. (New) The optical fiber coil according to claim 39, wherein the diameter of the covering of said first optical fiber is not more than 150 μm.
 61. (New) The optical fiber coil according to claim 39, wherein a covering of said first optical fiber is an ultraviolet-curing resin and the Young's modulus thereof is 0.1-20 kg/mm2.
 62. (New) The optical component according to claim 39, wherein the cladding diameter of said first optical fiber is not more than 100 μm.
 63. (New) The optical fiber coil according to claim 39, wherein said optical fiber coil comprises aligned windings.
 64. (New) The optical fiber coil according to claim 39, wherein a plurality of said optical fiber coils according to either of claim 1 are set in one housing case.
 65. (New) The optical fiber coil according to claim 39, wherein said first optical fiber is twisted by 90° or more per meter.
 66. (New) The optical fiber coil according to claim 39, wherein the shape of said optical fiber coil is noncircular.
 67. (New) The optical fiber coil according to claim 39, wherein a pull-out force of said first optical fiber is not more than 120 g.
 68. (New) The optical fiber coil according to claim 39, wherein a clearance of not less than 1 μm is present between a glass part and a covering part of said first optical fiber.
 69. (New) The optical fiber coil according to claim 39, said optical coil being a dispersion compensator for decreasing chromatic dispersion in a wavelength region of not less than the wavelength of 1.50 μm in an optical fiber transmission line, wherein said optical fiber coil is an optical fiber coil obtained by winding a long optical fiber having chromatic dispersion coefficient and chromatic dispersion slope of an opposite sign to that of an optical fiber forming said optical fiber transmission line, by a plurality of turns, and making the fiber coil in a bundled state while decreasing transmission loss increase in the wavelength region of not less than the wavelength of 1.50 μm due to the winding by 0.1 dB/km or more. 